You probably need an Apple ADB keyboard and mouse for it.

It's ADB, uses the same connector as SVideo cables, and it's chainable so the mouse plugs in the keyboard, and if you have a joystick you put it in the chain too.
Not sure about adapters.

I did a quick search for ADB to PS/2 or USB adapters and nothing really came up. Maybe I just didn't find it.

But 1 alternative option you do have, is to buy a PCI USB card and then you can use a USB mouse & keyboard instead - need to make sure you get one that supports the classic Mac OS though, but...

In order to load the drivers to get it working, you're probably going to have to take out the HDD and somehow connect it up to either a more modern Mac or a PC with Mac OS X or above in something like Parallels or VMWare. Then you'd have to drag the extensions across manually to the correct folder.

Basically it's doable, but a pain. Also you'd have to be aware that the USB won't function until the extension loads at boot, so best to turn off the option to check the disk if the system isn't shutdown properly or anytime that happens you'll get stuck. Alternatively you can also upgrade the RAM & install Mac OS X (though it won't perform as well as the classic Mac OS on that G3), which should support a PCI USB card - but you'd have to select the CD-ROM startup disk for Mac OS X from within Mac OS as the "hold down C at boot" shortcut obviously won't work with a USB keyboard.

Probably easier all around to find an old Apple ADB keyboard & mouse.

ADB to USB certainly does exist, in fact I'm using one now, typing on an Apple Extended Keyboard II on a Win10 PC. Back in the day there were various option that still turn up on auction sites, but I'm using a more recent homebrew-type device:

https://www.bigmessowires.com/usb-wombat/

Reason I chose this one is that it works both ways, ADB to USB but also USB to ADB (for exactly your situation).

That ADB wombat is a nice find.